Many composite materials are being reinforced with high strength, high modulus filaments such as boron or silicon carbide. The composite matrix materials provide high strength and stiffness while at the same time give reduced component weight.
U.S. Pat. No. 4,068,037 to DeBolt et al and assigned to the same assignee as this invention discloses means for making silicon carbide filament. In accordance with the DeBolt et al patent, a carbon filament is drawn from a supply reel and passed through a reactor chamber comprised generally of a closed tubular cylinder having oppositely disposed closed ends. In each of the closed ends, there is a central aperture which allows the carbon filament to pass into and out of the reactor on an uninterrupted basis. A mercury contact in each aperture allows that portion of the filament which is within the reactor to be raised in temperature by resistance heating. A number of ports in the sidewalls of the reactor tube allow chemicals to be injected into and withdrawn from the reactor chamber. Typically, a mixture of hydrogen and silanes are fed into the tubular cylinder. When the silanes come in contact with the heated filament core, a chemical vapor deposition process takes place and silicon carbide is deposited on the core.
DeBolt et al further teaches that it is advantageous to apply a surface layer of carbon-rich silicon carbide to the silicon carbide coating during the deposition process. The carbon-rich layer was shown to both improve tensile strength and decrease sensitivity to surface abrasion. The layer was approximately one micron thick and had a chemical composition which varied from pure carbon at the outer surface to silicon carbide at a depth of one micron. This carbon-rich coating both enhanced the strength of the filament and at the same time made the filaments easier to handle by the operator.
The carbon-rich filament of the DeBolt et al patent exhibits certain deficiencies. For example, when the filaments are fabricated into an aluminum composite, they do not wet easily in molten aluminum. Wetting can be accomplished at very high temperatures, but this degrades the strength of the filaments, presumably by chemical reaction between the aluminum and the carbonaceous surface. Aluminum alloys containing magnesium, nickel or titanium wet the filament at lower temperatures, but degrade their strength by chemical attack. In aluminum composites fabricated at lower temperatures, for example, by diffusion bonding, there is poor bonding between the filament and the matrix. In composites where the matrix material is titanium, the properties of the composite suffer because of mutually adverse interaction between the filament and the matrix. In matrix materials such as epoxy resins, it has also been difficult to provide good bonding between the carbonaceous surface and the resin.
With our invention, these problems are corrected. We apply a thin coating of silicon-rich silicon carbide to the silicon carbide substrate filament. The silicon-rich coating has a pure silicon exterior surface. The carbon silicon ratio C/Si increases as one proceeds into the interior of the coating and reaches stoichiometric proportion at or near the surface of silicon carbide substrate filament. The silicon coating simultaneously:
(a) reduces the notch sensitivity of SiC filament, thereby increasing the filament strength, PA1 (b) provides a surface that is readily wet by molten aluminum alloys, PA1 (c) provides a surface that is readily bonded to aluminum alloys during casting, diffusion bonding, or hot molding consolidation processes, PA1 (d) provides a surface that prevents excessive interdiffusion between aluminum alloys and the SiC filament during fabrication or consolidation, PA1 (e) provides a surface that can be transformed to SiO.sub.2 for enhanced bonding in glass and resin matrices and enhanced reactivity towards coupling agents.
The silicon-rich coating is applied to a silicon carbide filament. The methods and conditions under which the coatings are applied are unique to this invention.